The present invention relates to tarnish-resistant precious-metal alloys based on silver, gold, palladium and copper with additives of indium and zinc for dentistry. The alloys of the present invention are especially well suited for casting crowns and bridges which are to be veneered with plastic or which remain unveneered.
Alloys with a content of at least 75% gold and platinum metals have proven to be superior in dentistry technology for decades but are quite expensive. Alloys with 50-60% gold and additives of 5-10% palladium have also been known for many years but are likewise relatively expensive.
Considerably less expensive precious-metal alloys are known; i.e. with a gold content below 50%, palladium content of 5-30%, with a high silver content and with additives o indium, copper and zinc. However, these alloys differ from the alloys with a higher gold content by virtue of the fact that they normally exhibit a two-phase structure.
This second phase is always heavily enriched with palladium an indium, which results in a depletion of these elements in the silver-rich matrix. However, the tarnish resistance of silver-rich alloys depends to a great extent on its palladium and indium content, both of which suppress the tarnishing of the surface by sulfides.
The two-phase feature of the structure of these prior known alloys is furthermore associated with the disadvantage that their corrosion potential can differ greatly vis-a-vis different anions and complexing agents, so that local element formations occur, which results in greater corrosion. Moreover, since the palladium-rich phase is harder than the silver-rich phase an uneven surface can readily occur during the polishing of a crown or bridge (orange skin).
A further disadvantage is the reduced deformability (elongation) of two-phase alloys.